Screencasts and blog posts on workflow, productivity, tools, Mozilla and whatever else tickles my fancy.

The New Mercurial Workflow

There’s a good chance you’ve heard something about a new review tool coming to Mozilla and how it will change everything. There’s an even better chance you’ve stumbled across one of gps’ blog posts on how we use mercurial at Mozilla.

With mozreview entering beta, I decided to throw out my old mq based workflow and try to use all the latest and greatest tools. That means mercurial bookmarks, a unified mozilla-central, using mozreview and completely expunging mq from my workflow.

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How many tests are disabled?

tl;dr Look for reports like this in the near future!

At Mozilla, platform developers are culturally bound to tbpl. We spend a lot of time staring at those bright little letters, and their colour can mean the difference between hours, days or even weeks of work. With so many people performing over 420 pushes per day, all watching, praying, rejoicing and cursing, it’s paramount that the whole process operates like a well oiled machine.

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When would you use a Python mixin?

That’s not a rhetorical question. I’d like to know in which scenarios a mixin in python really is the best option. I can’t seem to think of any, but maybe I’m not thinking outside the box enough.

The basic idea of a mixin is to create a small re-usable class that can “plug-in” to other larger classes. From the wikipedia definition, a mixin is a way to compose classes together without using inheritance. The problem is unlike ruby, python mixins are a purely conceptual construct. Python mixins are inheritance (the only difference is that the class name usually contains ‘Mixin’). It is up to the developer to remember this, and to manually avoid all of the common pitfalls of multiple inheritance. This kind of defeats the whole purpose of the mixin in the first place. What’s more is that most people use python mixins improperly.

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Part 2: How to deal with IFFY requirements

My last post was basically a very long winded way of saying, “we have a problem”. It kind of did a little dance around “why is there a problem” and “how do we fix it”, but I want to explore these two questions in a bit more detail. Specifically, I want to return to the two case studies and explore why our test harnesses don’t work and why mozharness does work even though both have IFFY (in flux for years) requirements. Then I will explore how to use the lessons learned to improve our general test harness design.

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Part 1: Sharing code is not always a good thing

Dry versus Wet

As programmers, we are taught early on that code duplication is bad and should be avoided at all cost. It makes code less maintainable, reusable and readable. The DRY principle is very basic and fundamental to how most of us approach software design. If you aren’t familiar with the DRY principle, please take a minute to read the wikipedia page on it. The counterpart of DRY, is WET (write everything twice). In general, I agree that DRY is good and WET is bad. But I think there are a class of problems where the DRY approach can actually be harmful. For these types of problems, I will make a claim that a WET approach can actually work better.

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Add more mach to your B2G

Getting Started

tl;dr - It is possible to add more mach to your B2G repo! To get started, install pip:

$ wget -O - | python

Install b2g-commands:

$ pip install b2g-commands

To play around with it, cd to your B2G repo and run:

$ git pull                 # make sure repo is up to date
$ ./mach help              # see all available commands
$ ./mach help <command>    # see additional info about a command


Most people who spend the majority of their time working within mozilla-central have probably been acquainted with mach. In case you aren’t acquainted, mach is a generic command dispatching tool. It is possible to write scripts called ‘mach targets’ which get registered with mach core and transformed into commands. Mach targets in mozilla-central have access to all sorts of powerful hooks into the build and test infrastructure which allow them to do some really cool things, such as bootstrapping your environment, running builds and tests, and generating diagnostics.

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A Workflow for using Mach with multiple Object Directories

Mach is an amazing tool which facilitates a large number of common user stories in the mozilla source tree. You can perform initial setup, execute a build, run tests, examine diagnostics, even search Google. Many of these things require an object directory. This can potentially lead to some confusion if you typically have more than one object directory at any given time. How does mach know which object directory to operate on?

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We Are All Enablers

This probably isn’t a groundbreaking revelation to most Mozillians, but it’s something I haven’t quite managed to put into words until now.

On the Tools and Automation team at Mozilla, the easiest way to describe what we do is to say “We try to make the lives of others within the community easier”. This is a vague statement, but it’s about as close as we can get without writing a small dissertation.

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Running B2G unittests with Mach

Before now running ‘classic’ unittests (mochitest, reftest, xpcshell, etc.) on B2G emulators has been a massive pain. The new recommended way of running them is through mach.

  1. Update B2G repo if you haven’t already: git pull
  2. Configure an emulator: BRANCH=master ./ emulator
  3. Build: ./
  4. Run: ./mach mochitest-remote

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How to Push a Custom Test Command Line to Try


The steps in this article are no longer accurate. Pushing a custom command line is easier now. See here for more details.

Have you ever wanted to see the test results of a custom command line in try? Things like –test-manifest, –shuffle or –run-slower? Now you can! The process isn’t exactly optimized for the developer use case, but neither is it really difficult to do once you know how.

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